Gas turbine engines generally include a gas generator which comprises a compressor for compressing air flowing through the engine, a combustor in which fuel is mixed with the compressed air and ignited to form a high energy gas stream, and a turbine which includes a rotor for driving the compressor. Many engines further include a second turbine, known as a power turbine, located aft of the gas generator and which extracts energy from the gas flow to drive either a ducted fan or a propeller.
A recent improvement over the turbofan and turboprop engines described above is the unducted fan engine such as disclosed in U.S. patent application Ser. No. 437,923, now abandoned. In the unducted fan engine, the power turbine includes counterrotating rotors and turbine blades which drive counterrotating unducted fan blades radially located with respect to the power turbine. In order to achieve optimum performance, each of the unducted fan blades will have a variable pitch which requires a blade retention bearing to support each blade hub.
In conventional turboprop engines, the blade retention bearing is supported by a relatively deep cup-shaped structure. This is necessary in order to limit bearing deflections which are caused by the high centrifugal loads of the rotating blades. For example, hub ring depth to diameter ratios on the order of 2 to 1 or greater are known.
The radial location of the unducted fan blades with respect to the power turbine limits the bearing hub depth. The generic problem thus presented is how to support a plurality of variable pitch rotating blades with a relatively shallow hub support.